ClamSMTP is an SMTP filter that allows you to check for viruses using the
ClamAV anti-virus software. It accepts SMTP connections and forwards the
SMTP commands and responses to another SMTP server. The 'DATA' email body is
intercepted and scanned before forwarding. ClamSMTP aims to be lightweight,
reliable, and simple rather than have a myriad of options. It's written in C
without major dependencies. If you need more options then you could use
something big like AMaViS which is written in PERL and can do almost
anything.